Four kinds of core-shell structure acrylic impact modifiers (AIMs) with different rubber crosslinking densities were synthesized. The effects of the rubber crosslinking density of the AIMs on the crack initiation and propagation resistance and the mechanical properties of the AIM/poly(methyl methacrylate) (PMMA) blends were investigated, and we found that the maximum stress intensity factor, crack propagation energy, and Izod impact strength reached maximums when the appropriate rubber crosslinking density of AIM, 2.51 X 10(25) crosslinks/m(3), was adopted. Transmission electron microscopy photographs of the AIM/PMMA blends showed that the AIMs dispersed uniformly in the PMMA matrix. Meanwhile, through the analysis of optical photos and scanning electron microscopy of the impact fracture surface, we found that the deformation mechanism of the AIM/PMMA blends was local matrix shear yielding initiated by rubber particle cavitation of the AIM. The rubber of the AIM, whose crosslinking density was 2.51 x 10(25) crosslinks/m(3), was beneficial to the formation of intensive voids and initiated the local shear yielding of nearby modifiers of the PMMA matrix effectively in impact tests, which led to higher Izod impact strengths. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 2386-2393, 2010